1. Field of the Invention
The present invention relates to a hydraulic cylinder.
2. Description of the Related Art
A hydraulic cylinder is generally provided with a seal member for sealing an annular gap between a piston and a cylinder FIG. 5 is a schematic sectional view showing a sealing system provided with a hydraulic cylinder according to a conventional example.
The sealing system according to a conventional example shown in FIG. 5 seals an annular gap between two members (a piston 400 and a cylinder 500 in the illustrated example) that relatively perform reciprocating motion. The sealing system includes three seal members arranged in the order of a buffer ring 100, a rod seal 200, and a dust seal 300 from the side (O) on which oil is sealed toward the atmosphere side (A). The buffer ring 100 principally carries out a function to buffer pressure (hydraulic pressure) of oil to be sealed, the rod seal 200 principally carries out a function to prevent leakage of oil, and the dust seal 300 principally carries out a function to prevent intrusion of dust from outside.
In a hydraulic cylinder provided with a sealing system as described above, the piston 400 is inserted so as to have minute clearance S with respect to an inner circumferential surface of a shaft hole provided in the cylinder 500 and each seal member (the buffer ring 100, the rod seal 200 and the dust seal 300) is arranged in each annular groove provided on the inner circumferential surface of the shaft hole.
Here, in a hydraulic cylinder in which a piston and a cylinder relatively perform reciprocating motion at high speed like, for example, a breaker and which is used in an environment in which hydraulic pressure rises, cavitation occurs when oil enters clearance from an annular groove where a seal member is arranged. In the sealing system shown in FIG. 5, cavitation could occur in portions (three locations) denoted by X.
When such cavitation occurs, very high pressure is instantaneously created in the vicinity of the cavitation that occurs due, for example, to a cavitation shock wave originating from a burst of bubbles. Accordingly, there is a problem that losses of seal lips of the seal members occur scattered near the tips thereof and thus, sealing performance deteriorates. Though seal members are assumed to be replaced in a timely fashion as a consumable item in a sealing system used in a very severe environment like a breaker, endurance life of seal members is demanded to be made longer.